The first trimester lasts from the first day of your last menstrual period through 13 weeks and 6 days, making it just under 14 full weeks. That’s roughly three and a half months, though it won’t feel evenly paced. The early weeks often pass before you even know you’re pregnant, while the later weeks can feel like they stretch on forever.
How the Start Date Is Calculated
The clock starts earlier than most people expect. Doctors count pregnancy from the first day of your last menstrual period, not from the day you conceived. This is called gestational age. Since ovulation and conception typically happen about two weeks into your cycle, you’re already considered “two weeks pregnant” at the moment the egg is fertilized. Your actual fetal age, sometimes called conceptual age, runs about two weeks behind the gestational age used on your chart.
This means the first trimester spans roughly 14 weeks on the calendar, but your baby has only been developing for about 12 of those weeks. If the math feels confusing, you’re not alone. The system exists because most people can pinpoint when their last period started, while the exact day of conception is harder to confirm.
What Happens to the Baby in 14 Weeks
Development moves remarkably fast during the first trimester. This is the period when every major organ system takes shape, which is why it’s sometimes called the period of organogenesis.
Around week 3 (gestational age), sperm and egg unite in a fallopian tube to form a single-celled zygote. By week 4, that cluster of dividing cells has implanted in the uterine wall and begun differentiating into three layers. One layer becomes the brain and spinal cord, another forms the heart, bones, kidneys, and reproductive organs, and the third gives rise to the lungs and intestines.
By week 6, the neural tube along the baby’s back is closing, and the heart and other organs begin to form. Week 7 brings visible changes to the face: the beginnings of nostrils, the start of retinas, and tiny limb buds that will become arms and legs. By week 8, those limb buds have taken on paddle-like shapes. Over the remaining weeks of the trimester, fingers and toes separate, facial features become more defined, and the baby starts making small movements (though you won’t feel them yet).
The Symptom Timeline
Most pregnancy symptoms show up between weeks 5 and 7, when hormone levels begin rising sharply. The hormone hCG, which home pregnancy tests detect, follows a dramatic curve: levels average under 750 at week 4, jump to 200 to 7,000 by week 5, and can reach 32,000 to 210,000 between weeks 8 and 12. That steep climb is a major driver of nausea and fatigue.
Nausea and vomiting tend to peak between weeks 9 and 14, when 60 to 70 percent of pregnant people experience nausea and 30 to 40 percent are actively vomiting. This timing isn’t random. Symptoms are most intense precisely when the baby’s organ development is most vulnerable, and researchers believe nausea may serve a protective function by steering you away from foods that could contain harmful compounds. For most people, symptoms taper off as the second trimester begins, though the timeline varies.
Other common first trimester experiences include breast tenderness, frequent urination, food aversions, and deep fatigue. Weight gain is typically modest: most people gain 2 to 4 pounds total during the entire first trimester.
Miscarriage Risk by Week
The first trimester carries the highest risk of pregnancy loss, estimated at 10 to 20 percent of known pregnancies. But that number drops quickly as the weeks progress. Once a heartbeat is detected around 6 to 7 weeks, the risk of miscarriage falls to roughly 10 percent. By 8 weeks with a confirmed heartbeat, the chance of the pregnancy continuing rises to about 98 percent. At 10 weeks, it reaches 99.4 percent.
These numbers come from a study of over 300 women, so individual risk varies depending on age, health history, and other factors. Still, the pattern is consistent: each passing week with a healthy heartbeat significantly lowers the odds of loss.
Key Nutrients and Testing Windows
Because organ development is concentrated in the first trimester, folic acid is critical during this stretch. The CDC recommends 400 micrograms daily for anyone who could become pregnant, ideally starting at least one month before conception and continuing through the first three months. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily.
The first trimester also has a narrow window for certain prenatal screening. Between weeks 11 and 13, an ultrasound can measure fluid at the back of the baby’s neck, a screening called nuchal translucency that helps assess the likelihood of certain chromosomal conditions. This test can only be performed during that specific two-week window because the measurement becomes unreliable afterward.
When the Second Trimester Begins
You officially enter the second trimester at 14 weeks and 0 days. For many people, the transition brings noticeable relief: nausea fades, energy returns, and the risk of miscarriage drops to its lowest point. The baby’s organs are formed and now shift into a phase of growth and refinement. If the first trimester felt like an endurance test, the second is often where pregnancy starts to feel more manageable.